docs: add README for basic and vaulted checkout examples

[okxint]

What

Adds README.md to two examples that had no documentation at all:

• examples/primer-checkout-basic/
• examples/primer-checkout-vaulted/

Why

Both examples are linked from the root README.md but had zero documentation. A developer landing in either folder had no idea what the example does, when to use it, or how to run it.

What each README covers

Basic checkout:

• When to reach for the drop-in vs custom UI patterns
• End-to-end flow diagram (frontend → backend → Primer → processor)
• Why setAttribute() is required for the client token
• How to listen for checkout-complete
• Test card numbers for success/decline/3DS scenarios

Vaulted checkout:

• Business case for vaulting (returning customer conversion, subscriptions, one-click)
• How the vault flow differs between a first visit and a return visit
• Why customerId is required to link saved methods to a customer
• How to charge a vaulted payment method server-side after the initial checkout
• Links to the Vault API and Payments API for next steps

Both READMEs follow the format established by primer-checkout-themes/README.md.